libctf, link: add the ability to filter out variables from the link

The CTF variables section (containing variables that have no
corresponding symtab entries) can cause the string table to get very
voluminous if the names of variables are long.  Some callers want to
filter out particular variables they know they won't need.

So add a "variable filter" callback that does that: it's passed the name
of the variable and a corresponding ctf_file_t / ctf_id_t pair, and
should return 1 to filter it out.

ld doesn't use this machinery yet, but we could easily add it later if
desired.  (But see later for a commit that turns off CTF variable-
section linking in ld entirely by default.)

include/
	* ctf-api.h (ctf_link_variable_filter_t): New.
	(ctf_link_set_variable_filter): Likewise.

libctf/
	* libctf.ver (ctf_link_set_variable_filter): Add.
	* ctf-impl.h (ctf_file_t) <ctf_link_variable_filter>: New.
	<ctf_link_variable_filter_arg>: Likewise.
	* ctf-create.c (ctf_serialize): Adjust.
	* ctf-link.c (ctf_link_set_variable_filter): New, set it.
	(ctf_link_one_variable): Call it if set.

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